The official US government version of the death of Osama bin Laden has been thrown into doubt by a Navy SEAL who was on the scene and shot the leader of al Qaeda.

According to revelations in No Easy Day, the autobiography of a special forces trooper writing as "Mark Owen", bin Laden was unarmed, had no access to loaded weapons, and was shot on the threshold of the room where his body was found.

The official version is that bin Laden was killed by SEALS who burst into his room to be confronted by two women who stood in front of the terrorist mastermind to protect him.

The Pentagon said that the commandos pulled the women away and then shot bin Laden, hitting him once in the chest and once in the head.

But the author of the new book was the second man into bin Laden’s room on the third floor of his house in a compound in Abbottabad, Pakistan.

"Owen" wrote that a member of the SEAL Team 6 saw the terror leader put his head out of his room to look down the narrow stairwell the commandos were running up.

He said: "We were less than five steps from getting to the top when I heard suppressed shots. BOP. BOP. I couldn't tell from my position if the rounds hit the target or not. The man disappeared into the dark room."

Image Caption:Osama bin Laden was found living in a compound in Abbottabad, Pakistan

While various media outlets reported that bin Laden was armed, and showed no intention of going quietly, the soldier wrote that the notorious terror boss was already badly wounded when they followed him into his room.

Owen writes that as bin Laden lay dying he and another commando, "trained our lasers on his chest and fired several rounds. The bullets tore into him, slamming his body into the floor until he was motionless."

Copies of the book have been seen by journalists at the Associated Press and Huffington Post in the US.

In the account related by administration officials after the raid in Pakistan, the SEALs shot bin Laden only after he ducked back into the bedroom because they assumed he might be reaching for a weapon.

In the Owen version of events two weapons – neither of them with loaded magazines - were found in a corner of bin Laden’s room.

Owen wrote: "He hadn’t even prepared a defence. He had no intention of fighting. He asked his followers for decades to wear suicide vests or fly planes into buildings, but didn’t even pick up his weapon.

"In all of my deployments, we routinely saw this phenomenon. The higher up the food chain the targeted individual was, the bigger a pussy he was."

Image Caption:President Obama has come under fire for "claiming credit" for the operation

No Easy Day was due to be published on September 11 with profits from its sales going to military charities in the US.

It is now expected to hit book stores on September 4 after a surge of interest sparked by recent revelations.

The book is also reported to cast doubt on claims from the Obama administration that bin Laden’s body was treated with respect.

Owen claims that a fellow commando called "Walt" sat on the dead man’s chest in a cramped helicopter returning to its base after the raid.

He also said that there had been no "40 minute fire fight" at the compound which officials had first claimed.

There is a political significance to the timing of its publication – two months before a presidential election.

President Obama has been criticized for allegedly exploiting the successes of the armed forces of the US in combating terror.

A group claiming to be non-partisan called Special Ops OPSEC (Special Operations Operational Security) has launched a campaign with a 22-minute video attacking him for seeking to claim credit for long-planned CIA and special forces operations.

Ben Smith, a former SEAL, says on it: "Mr President, you didn’t kill Osama bin Laden, America did. The work that the American military has done killed Osama bin Laden."

According to the Huffington Post, Owen writes about sitting around a fire on the eve of the bin Laden operation with other SEALs joking about who would play them in the movie of the operation.

"…We’ll get Obama re-elected for sure. I can see him now, talking about how he killed bin Laden," Owen wrote.

"We all knew the deal. We were tools in the toolbox, and when things go well they promote it. They inflate their roles. But we should have done it. It was the right call to make. Regardless of the politics that would come along with it, the end result was what we all wanted," he adds.